1. Field of the Invention
The invention relates generally to radar systems having digital processing of the returned radar signals. In particular, the invention relates to PPI radar systems with digital processing of the video signal having apparatus or circuitry for rejecting interference caused by signals received directly from adjacent radar transmitters operating in the same frequency band.
2. Description of the Prior Art
Radar systems as used for pleasure boats or commercial navigation have usually employed the PPI mode of operation in which the returned radar echo signals are displayed along radial scan lines emanating from the center of the radar indicator display. Most commonly, analog signal processing techniques have been used in which the received radar echo signals are amplified and converted to a baseband or video signals for modulating the intensity of the cathode ray tube display device.
A number of problems have become evident in such systems. First, because the received signals were used directly to modulate the beam intensity of the cathode ray tube, dim displays were presented at short radar ranges due to the high writing rate or velocity of the cathode ray tube beam upon the phosphor screen.
A second problem with such radar systems was interference caused by the reception of signals transmitted directly from other nearby radar transmitters operating in the same frequency band. This type of interference appeared as intense spiral arms radiating outward from the center of the radar presentation. Frequently this type of interference was so strong as to completely obliterate many targets of interest. The problem was particularly bothersome in a harbor navigation situation where many other radar transmitters may be expected to be operating but in which an accurate radar presentation is needed to avoid collision.
To alleviate the brightness problem, radar systems have been constructed in which the video signal is first digitized and then processed such as by storing each return signal then playing it back at a slower rate than that at which it was read in to effectively decrease the required writing rate upon the system's cathode-ray tube. However, no provisions were made for eliminating interference from adjacent radar transmitters.
In analog systems before the advent of digital video signal processing, a number of different techniques were employed for attempting to reduce the effects of interference caused by adjacent radar transmitters. Sector blanking techniques were used in which a device turned off the receiver and/or transmitter when the antenna was pointing to a specific sector in which was located an adjacent radar transmitter. Although such devices eliminated interference from equipment falling within the specified sector, all other targets were also lost. Pulse blanking circuits were used to effectively blank out the video signal during a time that an interference pulse was expected. Knowledge of when the interfering pulse is expected was required. Such knowledge had to be transmitted from the remote transmitting location. Although such systems could be used in land based situations, pulse blankers are generally inapplicable to seagoing situations. PRF discriminators were used employing a delay line and coincidence circuit to eliminate all incoming signals that don't have the same PRF as the present radar. Highly accurate and stable delay lines were required to maintain registration between adjacent sweep receive times. Most commonly used were various filter techniques, all of which substantially reduced the performance of the radar receiver and none of which completely eliminated the interference problem.